This invention relates to a coaxial conductor interconnection wiring board and a process for producing the same.
With the development of electronic devices, wiring boards having a very high wiring density have been required. As such wiring boards having a very high wiring density, there are known wiring boards using wires as a necessary wiring pattern (hereinafter referred to as "multi-wire wiring board", e.g. Japanese Patent Examined Publication No. 45-21434) and multi-layer printed wiring boards. In highly densified wiring boards, the crosstalk noise among neighboring wires or signal conductor lines becomes an important problem. In order to solve such a problem, there are proposed various processes for shielding wiring boards, for example, a process for making signal conductor lines and a grounding pattern by build-up method (Japanese Patent Examined Publication No. 58-54520), a process for using an electroconductive coating film as a shield (Japanese Patent Unexamines Publication No. 51-71961), a process for providing a conductive shield surrounding insulated wire conductors by plating method (U.S. Pat. No. 4,646,436), etc. But these processes are insufficient for solving the problem of crosstalk noise.
On the other hand, in order to improve these shielding processes, there are proposed processes for using a so-called coaxial conductor covered with a conductive shield on an outer insulating layer surrounding a signal conductor (U.S. Pat. Nos. 4,679,321 and 4,743,710). When such coaxial conductors are used, connections of a central signal conductor and a conductive shield of the coaxial conductor with other conductors become a problem. According to U.S. Pat. No. 4,743,710, there are proposed two processes. According to a first process (shown in FIGS. 1A to 1E of said U.S. Patent), it is necessary to remove an electroconductive substance, and a conductive shield and insulation of the coaxial conductor using a focussed laser at a portion to be connected, while retaining only a central signal conductor. But when the laser energy is made small so as not to remove the central signal conductor of coaxial conductor, removal of the portions to be removed cannot be carried out completely. In contrast, when the laser energy is made large so as to remove the portions to be removed completely, the central signal conductor is also vaporized or damaged. Thus, it is difficult to determine conditions satisfying both requirements and to maintain such a laser energy constantly. Further, according to the description at lines 58 to 67 on column 3 of said U.S. Patent, the central signal conductor reflects the CO.sub.2 laser energy and other organic materials can be removed. But since the organic ground layer (here, an electroconductive paste is imaged) has high electro-conductivity, the shielding effect becomes small. According to a second process (shown in FIGS. 2A to 2F of said U.S. Patent), it is necessary to etch back selectively the conductive shield of coaxial conductor exposed to an inner wall of hole. In order to attain such a selective etch back, it is necessary to use different materials for the conductive shield and the central signal conductor of the coaxial conductor. Further, this means that it is difficult to insulate the inner portion of hole including the selectively etched back portion and a surface of the ground layer. When different materials are used for the conductive shield and the central signal conductor of coaxial conductor, the decay of signal of central signal conductor becomes larger in the case of using a poor electroconductive material for the central signal conductor of coaxial conductor, while the shielding effect becomes smaller in the reverse case. Further, when the conductive shield of coaxial conductor is selectively etched back, very small vacant spaces are formed in the etched back portions. It is difficult to fill such vacant spaces with a resin solution, and even if filled with the resin solution, generation of undesirable voids is inevitable due to vaporization of a solvent contained in the resin solution.